This application requests financial support of the Third International Workshop on Opportunistic Protists. Three separate workshops are proposed for this meeting: Workshop I - Pneumocystis, Workshop II - Cryptosporidium and microsporidia, and Workshop III - Toxoplasma. The organisms to be discussed are opportunistic pathogens responsible for much morbidity and mortality in AIDS patients and in other immunocompromised hosts. Despite the medical, economic, and environmental importance of these pathogens, there is relatively little research on their biology, cultivation and modes of pathogenesis. New and improved therapies for controlling and eliminating these infections are only slowly being developed. To increase the quality and quantity of research on these organisms, the organizers propose to bring together biologists with molecular biologists, cell biologists, biochemists and immunologists with interests in parasitology. An important goal is to understand better the clinical problems caused by these pathogens and to set up fruitful collaborations and avenues of communications. Information on the state of research on these organisms is to be rapidly and efficiently disseminated to investigators working on these pathogens. An additional aim is to encourage young investigators to devote their talents to the study of these organisms to maintain a pool of knowledgeable researchers. These workshops will be held within the duration of the meeting of the 1994 Society of Protozoologists at Cleveland State University. Each of the workshops will be divided into separate sections to be held from Friday, June 24, through Monday, June 27. Workshop I (Pneumocystis) presents six sessions: (1) host interactions; (2) recent advances in biochemistry and cell biology; (3) recent advances in molecular biology; (4) pathogenesis; (5) chemotherapeutic agents and metabolism; and (6) a round table on taxonomy. Workshop II covers Cryptosporidium and microsporidia and is divided into four sessions: (1) microsporidial infections with emphasis on AIDS; (2) Cryptosporidium epidemiology and water-borne transmission; (3) Cryptosporidium pathogens and host-parasite interactions; and (4) Cryptosporidium and microsporidia cellular and molecular biology. The third workshop on Toxoplasma will have invited speakers, 10 minute talks, and poster sessions. No specific topics are provided.